1. Field of the Invention
Embodiments of the invention relate to a display device, and more particularly to a mother substrate for a liquid crystal display (LCD) device and a method of fabricating a liquid crystal display device.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are widely used as monitors for notebook computers and desktop computers and televisions because of their high resolution, high contrast ratio, color rendering capability and superior performance in displaying moving images. An LCD device relies on the optical anisotropy and polarizing properties of liquid crystal to produce an image. An LCD device includes a liquid crystal panel including two substrates and a layer of liquid crystal molecules between the two substrates. An electric field generated between the two substrates controls an alignment direction of liquid crystal molecules to produce differences in transmittance. More specifically, the LCD device displays images by producing differences in transmittance of light from a backlight unit under the liquid crystal panel.
An active matrix type liquid crystal display (AM-LCD) device has a plurality of pixels that collectively display images. The pixels are arranged in matrix. Each of the pixels contains a thin film transistor used as a switching element.
FIG. 1 is a schematic block diagram of a liquid crystal display device according to the related art. As shown in FIG. 1, the liquid crystal display device includes a liquid crystal panel 10 and a driving circuit 20 for supplying signals to the liquid crystal panel 10. Although not shown in FIG. 1, the liquid crystal panel 10 includes first and second substrates and a layer of liquid crystal molecules between the first and second substrates. The first substrate, which is referred to as an array substrate, includes gate lines 12 and data lines 14 that cross each other to define a plurality of pixel regions “P” in a matrix. Within each one of the pixel regions “P,” a thin film transistor (TFT) “T” is near each crossing of the gate lines 12 and the data lines 14 and connected to the gate lines 12 and the data lines 14. A pixel electrode is connected to the thin film transistor (TFT) “T” in each pixel region “P.” In addition, the second substrate, which is referred to as a color filter substrate, includes a color filter layer (not shown) and a common electrode (not shown). The common electrode faces the pixel electrode across the layer of liquid crystal molecules (not shown). As a result, the common electrode, the pixel electrode and the layer of liquid crystal molecules constitute a liquid crystal capacitor “Clc” connected to the TFT “T”.
A driving circuit 20 includes a timing controller 22, a gate driver 24 and a data driver 26. Although not shown in FIG. 1, the driving circuit 20 may further include an interface, a reference voltage generator and a source voltage generator. The interface transmits source signals from an external driving system (not shown), such as a personal computer, to the timing controller 22. The timing controller 22 generates gate control signals for the gate driver 24, and data control signals and data signals for the data driver 26. The gate driver 24 and the data driver 26 are attached to two adjacent sides of the liquid crystal panel 10 through a tape carrier package (TCP) to the gate lines 12 and the data lines 14, respectively. In response to the gate control signal, the gate driver 24 generates a gate signal that sequentially enables the plurality of gate lines 12 in each frame. The TFT “T” connected to each gate line 12 is turned on/off according to the gate signal. In addition, the data driver 26 selects a reference voltage in response to the data control signal and the data signal and supplies the reference voltage to the data lines 14.
When the TFT “T” is turned on in response to the gate signal of the gate driver 24, the data signal of the data driver 26 is transmitted to the pixel electrode through the TFT “T,” and the liquid crystal molecules are driven by an electric field between the pixel electrode and the common electrode. The reference voltage generator generates reference voltages for a digital to analog converter (DAC) of the data driver 26, and the source voltage generates a source voltage for elements of the driving circuit 20 and a common voltage for the common electrode of the liquid crystal panel 10.
The TFT for an LCD device may be classified into an amorphous silicon TFT or a polycrystalline silicon TFT depending on the crystalline state of the semiconductor material. When an amorphous silicon TFT is used, the gate driver 24 and the data driver 26 are formed to be separated from the liquid crystal panel 10 and are attached to the liquid crystal panel 10 through a tape automated bonding (TAB) method so as to connected them to the gate lines 12 and the data lines 14, respectively.
A gate in panel (GIP) type LCD device has been researched and developed in which a either a portion of the gate driver or all of the gate driver is integrated into a liquid crystal panel using an amorphous silicon TFTs to reduce fabrication costs and simplify fabrication processes. In general, a gate driver includes a shift register unit and a level shifter unit. The shift register unit includes a plurality of flip-flops in which each is a bistable device that outputs a signal according to selection of set and reset signals. The level shifter unit amplifies a level of the output signal. In a GIP type LCD device, at least the shift register unit including a plurality of shift register stages is integrated onto a first substrate. The plurality of shift register stages correspond to a plurality of gate lines on the first substrate. Either a portion or the entire gate driver is integrated onto the first substrate so as to be concurrently fabricated while an array element is formed on the first substrate. As a result, additional fabrication cost and an additional fabrication process for the other portion of the gate driver or the entire gate driver are not required.
A GIP type LCD device has some disadvantages. The gate driver is only completed when the fabrication process for the first substrate is finished. Accordingly, the gate driver can not be inspected before the fabrication process for the LCD panel is finished, i.e., no midterm inspection of the gate driver exists. As a result, it is hard to inspect the properties of the gate driver, such as a normal operation of the plurality of shift register stages, reliability under a long time period operation and deterioration, before the LCD panel is fabricated.
The gate driver, including the plurality of shift register stages, can also be inspected after the fabrication process of the LCD panel, which includes a fabrication step of the first substrate having an array element, a fabrication step of the second substrate having a color filter layer and an attaching step of the first and second substrates with an interposed liquid crystal layer, is finished. However, since a process of removing laminated layers of the gate driver is required to expose the plurality of shift register stages, the inspected LCD panel can not be used as a product any more. Accordingly, fabrication cost increases and production yield decreases.